The normal riding mode of existing tricycles ridden by children is implemented with single front wheel and double rear wheels, where the single front wheel is a steering wheel which may be controlled via a handlebar to control steering. For sake of riding safety, the riding mode of some tricycles is implemented with double front wheels and single rear wheel. For example, the Chinese patent application CN 101830264A discloses a novel front-mounted type tricycle vehicle, including a novel front-mounted type motor tricycle, a novel front-mounted type electric tricycle, and a novel front-mounted type tricycle. The tricycle vehicle has the structure of two front wheels and one rear wheel. A front fork portion comprises a direction control lever, a left-and-right eccentric steering mechanism, a front fork fixing frame, a damping device, a left wheel and a right wheel. The direction control lever has a pivot point in its middle, and is coupled to the front fork fixing frame by a pivot shaft. The front fork fixing frame is coupled and integrated together with the ridge of the tricycle body. The rear wheel is disposed under the crossbar of the tricycle body. A front saddle and a back saddle are disposed above the rear wheel. For another example, the Chinese patent application CN 1572644A discloses a three-wheel rolling vehicle with a front two-wheel steering mechanism, the vehicle comprising a frame, a handlebar, a rear wheel rotating with respect to the frame, and a steering assembly which operates two front wheels. Two horizontal crossbars are disposed, and coupled to side tubes via hinges and coupled to the frame via additional hinges. The side tubes rotatably support steering tubes. All these existing tricycles have only one riding mode, either a mode of single front wheel and double rear wheels or a mode of double front wheels and single rear wheel. It is not possible to simultaneously have two riding modes implemented on the same tricycle, not to mention to switch between two riding modes. In other words, if riders are to enjoy the fun of two riding modes, they have to own two tricycles having different riding modes, which is not convenient for the riders.